When publishers think about ad blocking, they fixate on how the tech hurts their ad revenue. But fewer are thinking about how it could hurt their ability to collect data about readers — and that’s a major long-term issue

Most ad blockers, including Adblock Plus, work by preventing sites from loading elements from certain domains and subdomains. And while that blocks the ads themselves, it also blocks tracking cookies, which come from ad networks but also and data management platforms such as BlueKai and Krux. That means that readers running ad blockers are invisible when it comes to publishers’ data collection.

“It is for sure a concern. We collect valuable first-party data segments from sure visitors. The context of the site defines a key affinity of the consumer, a key interest-based data point,” said Evolve Media president Brian Fitzgerald.

For publishers with loyal audiences, data can be a major differentiator. Knowing that a particular reader visits a tech site multiple times a day, for example, says a lot about both that reader’s relationship with a site and that reader’s overall interest in a topic. Both are valuable insights for advertisers, which are always on the hunt for more info about the interests and habits of the people they advertise to. Publishers hope that by collecting and packaging that data, they can fetch higher premiums on their CPMs.

“We feel that these context-oriented, interest-based data segments are some of the cleanest, purest, most valuable first-party data segments out there,” Fitzgerald said.

That kind of data comes easy to big platforms such as Facebook and Google, which are flush with data and act as walled gardens when it comes to data. But for publishers, which are comparatively late to data collection, those sorts of insights are harder to come by. And ad blocking makes it that much harder.

Blocked cookies may not matter by themselves, but their losses add up. At recent poll from the Reuters Institute for the Study of Journalism found that 47 percent of people polled in the U.S. and 39 percent in the U.K. said that they run ad blocking software. AdBlock Plus, the most popular of the tools, says it has averaged 2.3 million downloads a week since 2013. PageFair, which helps publishers subvert ad blockers, estimates that the global ad blocking population is over 144 million monthly users.

Still, most publishers are preoccupied by the revenue side of the ad blocking equation — and for good reason. While lost data might hurt publishers in the long run, the bigger short-term problem is all the money publishers are losing off blocked ads. Ad blocking rates can run as high as 50 percent for gaming sites, and 25 percent for tech sites, according to PageFair.

“Our focus is the potential loss of ad revenue. While the disruption in data collection is part of the story, I don’t think it will be the driver in finding solutions,” said CBS Interactive chief revenue officer Dave Morris.